Optical Observations of SAX J1808.4-3658 During Quiescence

TL;DR

This study combines optical and X-ray observations of SAX J1808.4-3658 during quiescence to model the system's irradiation and constrain its binary parameters, suggesting an active radio pulsar mechanism and a massive neutron star.

Contribution

It provides the first detailed optical modeling of SAX J1808.4-3658 during quiescence, revealing enhanced irradiation likely from pulsar activity and constraining the system's inclination and component masses.

Findings

Donor star's face is significantly irradiated, exceeding X-ray luminosity.

Binary inclination constrained between 36 and 67 degrees.

Neutron star mass estimated to be >2.2 solar masses.

Abstract

We observed the accreting millisecond pulsar SAX J1808.4-3658 with Gemini-South in g' and i' bands, nearly simultaneous with XMM-Newton observations. A clear periodic flux modulation on the system's orbital period is present, consistent with the varying aspect of the donor star's heated face. We model the contributions of a disk and donor star to these optical bands. To produce the observed modulation amplitudes, we conclude that the donor must be irradiated by an external flux 2 orders of magnitude greater than provided by the measured X-ray luminosity. A possible explanation for this irradiation is that the radio pulsar mechanism becomes active during the quiescent state as suggested by Burderi et al., with relativistic particles heating the donor's day-side face. Our modelling constrains the binary inclination to be between 36 and 67 degrees. We obtain estimates for the pulsar mass of >2.2 solar masses (although this limit is sensitive to the source's distance), consistent with the accelerated NS cooling in this system indicated by X-ray observations. We also estimate the donor mass to be in the range of 0.07-0.11 solar masses, providing further indications that the system underwent non-standard binary evolution to reach its current state.